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POWER TRANSISTORS
Power transistors are devices that have controlled turn-on and turn-off
characteristics. These devices are used a switching devices and are operated in the
saturation region resulting in low on-state voltage drop. They are turned on when a
current signal is given to base or control terminal. The transistor remains on so long as
the control signal is present. The switching speed of modern transistors is much higher
than that of thyristors and are used extensively in dc-dc and dc-ac converters. However
their voltage and current ratings are lower than those of thyristors and are therefore used
in low to medium power applications.
Power transistors are classified as follows
 Bipolar junction transistors(BJTs)
 Metal-oxide semiconductor filed-effect transistors(MOSFETs)
 Static Induction transistors(SITs)
 Insulated-gate bipolar transistors(IGBTs)
BIPOLAR JUNCTION TRANSISTORS
The need for a large blocking voltage in the off state and a high current carrying
capability in the on state means that a power BJT must have substantially different
structure than its small signal equivalent. The modified structure leads to significant
differences in the I-V characteristics and switching behavior between power transistors
and its logic level counterpart.
POWER TRANSISTOR STRUCTURE
If we recall the structure of conventional transistor we see a thin p-layer is
sandwiched between two n-layers or vice versa to form a three terminal device with the
terminals named as Emitter, Base and Collector.
The structure of a power transistor is as shown below
Collector
Base
Collector
Base
npn BJT
pnp BJT
Emitter
Emitter
Base
10m
Base
5-20m
Thickness
50-200m
(Collector drift
region)
250m
Emitter
n
+
19
-3
10
cm
16
p
10
–
10
+
10
n
n
cm
14
cm
19
cm
-3
-3
-3
Collector
Fig. 1: Structure of Power Transistor
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The difference in the two structures is obvious.
A power transistor is a vertically oriented four layer structure of alternating p-type
and n-type. The vertical structure is preferred because it maximizes the cross sectional
area and through which the current in the device is flowing. This also minimizes on-state
resistance and thus power dissipation in the transistor.
The doping of emitter layer and collector layer is quite large typically 1019 cm-3.
A special layer called the collector drift region (n-) has a light doping level of 1014.
The thickness of the drift region determines the breakdown voltage of the
transistor. The base thickness is made as small as possible in order to have good
amplification capabilities, however if the base thickness is small the breakdown voltage
capability of the transistor is compromised.
Practical power transistors have their emitters and bases interleaved as narrow
fingers as shown. The purpose of this arrangement is to reduce the effects of current
crowding. This multiple emitter layout also reduces parasitic ohmic resistance in the base
current path which reduces power dissipation in the transistor.
Fig. 2
STEADY STATE CHARACTERISTICS
Figure 3(a) shows the circuit to obtain the steady state characteristics. Fig 3(b)
shows the input characteristics of the transistor which is a plot of I B versus VBE . Fig 3(c)
shows the output characteristics of the transistor which is a plot I C versus VCE . The
characteristics shown are that for a signal level transistor.
The power transistor has steady state characteristics almost similar to signal level
transistors except that the V-I characteristics has a region of quasi saturation as shown by
figure 4.
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Quasi-saturation
- 1/Rd
Hard
Saturation
Second breakdown
iC
IB5>IB4,etc.
IB5
IB4
IB3
Active region
Primary
breakdown
IB2
IB1
0
I B<0
IB=0
IB=0
BVSUS
vCE
BVCEO
BVCBO
Fig. 4: Characteristics of NPN Power Transistors
There are four regions clearly shown: Cutoff region, Active region, quasi
saturation and hard saturation. The cutoff region is the area where base current is almost
zero. Hence no collector current flows and transistor is off. In the quasi saturation and
hard saturation, the base drive is applied and transistor is said to be on. Hence collector
current flows depending upon the load. The power BJT is never operated in the active
region (i.e. as an amplifier) it is always operated between cutoff and saturation. The
BVSUS is the maximum collector to emitter voltage that can be sustained when BJT is
carrying substantial collector current. The BVCEO is the maximum collector to emitter
breakdown voltage that can be sustained when base current is zero and BVCBO is the
collector base breakdown voltage when the emitter is open circuited.
The primary breakdown shown takes place because of avalanche breakdown of
collector base junction. Large power dissipation normally leads to primary breakdown.
The second breakdown shown is due to localized thermal runaway. This is
explained in detail later.
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